okay, if you only have a few plants in these small tanks, co2 really isn't necessary, and if you do add co2 to these small tanks, lighting needs to increase too, photosynthesis(lighting) and co2 go hand in hand, to have a healthy planted tank they need to keep each other in check. So fish addict, if you have smaller tanks with only a few plants, no need to add co2, again, get some good test kits to watch oxygen and co2, as well as nutrients like phosphate and iron, as this is the only way to know for sure. But on the other hand, I have yet to talk to someone who overdosed a tank with co2 on a diy system, at max they only put out 1 to 1.5 bubbles a second, and if your diffusing them with an airstone I don't think you can, so if you want to experiment to see the difference, go ahead, the chances of harming your fish are slim to none. If your really unsure, research some more, get a good book, and make sure you know the tell-tale signs of co2 excess and deprivation. Good luck!